Q. 47 The genes for microRNA (miRNA) in plants are usually transcribed by
(A) RNA polymerase I
(B) RNA polymerase II
(C) RNA polymerase III
(D) RNA polymerase IV
Genes for microRNA (miRNA) in plants are primarily transcribed by RNA polymerase II, making option (B) the correct answer. This process produces primary miRNA transcripts (pri-miRNAs) that fold into hairpin structures for further processing into mature miRNAs, crucial for gene regulation in plant development. Understanding polymerase roles helps in mastering molecular biology concepts for exams like NEET or CSIR NET.
Correct Answer
Answer: (B) RNA polymerase II
In plants, miRNA genes function like protein-coding genes and are transcribed by RNA polymerase II into long pri-miRNAs capped at the 5′ end and polyadenylated at the 3′ end. These transcripts are processed by Dicer-like enzymes into mature miRNAs that guide RNA-induced silencing complexes (RISC). This Pol II dependency allows precise regulation similar to mRNA transcription, confirmed across plant studies.
Option Explanations
RNA Polymerase I
RNA polymerase I transcribes ribosomal RNA (rRNA) precursors, such as the 45S pre-rRNA in plants, essential for ribosome biogenesis but unrelated to small non-coding RNAs like miRNAs. It operates in the nucleolus and handles high-volume rRNA synthesis, not gene-specific regulation via miRNAs.
RNA Polymerase II
Correct. RNA polymerase II handles most plant miRNA genes, producing pri-miRNAs from MIRNA loci, often intergenic or intronic. Evidence from inhibitor studies (e.g., α-amanitin sensitivity) and promoter analysis shows Pol II specificity, distinguishing it from Pol III small RNAs.
RNA Polymerase III
RNA polymerase III transcribes small RNAs like tRNAs, 5S rRNA, and U6 snRNA, but not miRNAs, as pri-miRNAs exceed Pol III’s typical short transcript length and lack Pol III termination signals (e.g., U4 stretches). Early assumptions linked Pol III to miRNAs due to size, but direct evidence disproves this.
RNA Polymerase IV
RNA polymerase IV, plant-specific, transcribes non-coding RNAs for RNA-directed DNA methylation (RdDM) and heterochromatin silencing, targeting repetitive DNA rather than miRNA genes. It works with Pol V in silencing pathways, not miRNA biogenesis.
